Conventionally, in order to obtain a high-performance magnet having a dramatically enhanced coercive force, the following art has been proposed (e.g., in Patent Document 1) by the applicant of this patent application. The art in question discloses: to contain in a processing box neodymium-iron-boron sintered magnets and evaporating materials containing at least one of dysprosium (Dy) and terbium (Tb) at a distance from each other; to heat the processing box in a vacuum atmosphere to thereby evaporate the evaporating materials; to adjust the amount of supply of the evaporated metal atoms to the surfaces of the sintered magnets so that the metal atoms get adhered; and to perform the processing treatment to diffuse the adhered metal atoms into the grain boundaries and/or grain boundary phases of the sintered magnets so that a thin film made up of the metal evaporating material is not formed on the respective surfaces of the sintered magnets (vacuum vapor processing).
In the art of the above-mentioned Patent Document 1, as the evaporating materials, small particles, for example, were used so that they can be disposed around the sintered magnets that have been disposed inside the processing box. When the evaporating materials of this kind are used, the volumetric occupancy becomes large and, as a result, the amount of charging of magnets into the processing box cannot be increased. There was therefore a disadvantage in that the cost becomes higher for the above-mentioned processing treatment. In addition, there is another disadvantage in that the work of manually disposing small particles of evaporating materials into the processing box together with the sintered magnets is troublesome.
As a solution, the applicant of the present patent application has proposed to contain, inside the processing box, plate-shaped evaporating materials and sintered magnets by vertically stacking them while interposing spacers thereby preventing them from coming into contact with one another and thereby allowing for the metal atoms to pass therethrough (see Japanese Patent Application No. 2008-41555).
As a method of manufacturing a thin plate of Dy or Tb, it is considered to melt the ingots of Dy or Tb and cast them into slabs, e.g., in an inert gas atmosphere, and then subject them to rolling work. However, since Dy and Tb have high melting points and are extremely active, they react with the furnace materials or casting molds. It is therefore difficult to melt and cast them into slabs without inclusion of impurities therein. Even conceding that the melting and casting into slabs were possible, they have hexagonal lattice structure and thus are poor in workability. In addition, in order to roll them into thin plates, it becomes necessary to subject them to heat treatments in an inert gas for several times for annealing during the processing treatments. There was therefore a problem in that the manufacturing costs of the plate-like evaporating materials rapidly rise.